Many vertebrates, including humans, lack the ability to manufacture a number of amino acids and therefore require these amino acids in their diet. These are called essential amino acids. Grain-derived foods or feed, however, are deficient in certain essential amino acids, such as lysine, the sulfur-containing amino acids methionine and cysteine, threonine and tryptophan. For example, in corn (Zea mays L.) lysine is the most limiting amino acid for the dietary requirements of many animals, and soybean (Glycine max L.) meal is used as an additive to corn-based animal feeds primarily as a lysine supplement. Often microbial-fermentation produced lysine is needed for such supplementation. Thus, an increase in lysine content of either corn or soybean would reduce or eliminate the need to supplement mixed grain feeds with lysine produced via fermentation.
Furthermore, in corn the sulfur amino acids are the third most limiting amino acids, after lysine and tryptophan, for the dietary requirements of many animals. Legume plants, however, while rich in lysine and tryptophan, have low sulfur-containing amino acid content. Therefore, the use of soybean meal to supplement corn in animal feed is not satisfactory. An increase in the sulfur amino acid content of either corn or soybean would improve the nutritional quality of the mixtures and reduce the need for further supplementation through addition of more expensive methionine.
One approach to increasing the nutritional quality of human foods and animal feed is to increase the production and accumulation of specific free amino acids via genetic engineering of the biosynthetic pathway of the essential amino acids. Biosynthetically, lysine, threonine, methionine, cysteine and isoleucine are all derived from aspartate. Regulation of the biosynthesis of each member of this family is interconnected (see FIG. 1). The organization of the pathway leading to biosynthesis of lysine, threonine, methionine, cysteine and isoleucine indicates that over-expression or reduction of expression of genes encoding, inter alia, aspartic semialdehyde dehydrogenase, homoserine kinase, diaminopimelate decarboxylase, cysteine synthase and cystathionine β-lyase in corn and soybean could be used to alter levels of these amino acids in human food and animal feed. However, few of the genes encoding enzymes that regulate this pathway in plants, especially corn and soybeans, are available. Accordingly, availability of nucleic acid sequences encoding all or a portion of these enzymes would facilitate development of nutritionally improved crop plants.